There are applications in the fiber optics field in which a high power, low noise, broadband light source is of particular interest. For example, efforts are now being made toward spectral slicing wherein a common light source is used to generate a multitude of wavelength division multiplexed (WDM) signals. Such an application thus has the potential for replacing many lasers with a single light source. Other applications include, but are not limited to, frequency metrology, device characterization, dispersion measurements made on specialty fibers, and the determination of transmission characteristics of gratings. All of these various diagnostic tools, as well as many other applications, may be greatly enhanced by the availability of such a broadband source.
In general, continuum generation involves the launching of relatively high laser powers, typically in the form of optical pulses, into an optical fiber, waveguide or other microstructure, wherein the laser pulse train undergoes significant spectral broadening due to nonlinear interactions in the fiber. Previous efforts at continuum generation, typically performed using light pulses having durations on the order of picoseconds (10−12 sec) in kilometer lengths of fiber, have unfortunately shown degradation of coherence in the generation process. In particular, additional noise has been found to be introduced into the system during the spectral broadening aspect of the process.
Continuum light of wavelengths spanning more than one octave (variously referred to in the art as “supercontinuum”) have been generated in microstructured and tapered optical fibers by launching light pulses having durations on the order of femtoseconds (10−15 sec) into the ends of such microstructured or tapered fibers. The extreme spectra thus produced are useful, for example, in measuring and stabilizing pulse-to-pulse carrier envelope phase, as well as in high-precision optical frequency combs. Efforts at modeling the continuum in microstructured fibers based on a modified nonlinear Schrodinger equation (NLSE) have been aimed at understanding the fundamental processes involved in the spectrum generation, and show that coherence is better maintained as the launched pulses are shortened in duration from the order of picoseconds to femtoseconds.
A relatively new type of germanium-doped silica fiber with low dispersion slope and a small effective area, referred to hereinafter as “highly nonlinear fiber”, or HNLF, has recently been developed for use as the waveguiding medium in an optical continuum source. Although the nonlinear coefficients of HNLF are still smaller than those obtained with small core microstructured fibers, the coefficients are several times greater than those of standard transmission fibers, due to the small effective area of HNLF. Supercontinuum generation using an HNLF and a femtosecond fiber laser has been previously reported in the literature. U.S. Pat. No. 6,775,447 issued to J. W. Nicholson et al. on Aug. 10, 2004 discloses an HNLF supercontinuum source formed from a number of separate sections of HNLF fiber that have been fused together, each having a different dispersion value at the light source wavelength and an effective area between five and fifteen square microns. In its more general form, “highly nonlinear optical waveguide” can be defined to include various optical media other than fiber, such as optical waveguides formed on a substrate and the like. For the purposes of the present invention, the term “highly nonlinear waveguide” is defined as a waveguide in which the nonlinear length is at least ten times shorter than the dispersion length (see, “Nonlinear Fiber Optics”, G. P. Agrawal), where “nonlinear length” is defined as the inverse of the peak power of the input pulse multiplied by the waveguide nonlinear coefficient, and “dispersion length” is defined as the square of the input pulse width divided by the magnitude of the waveguide dispersion parameter β2.
In some applications, it is desirable to generate light beyond the wavelength edges of a given continuum (in most cases, such generation within only a narrow wavelength range is required). An “edge” of the continuum, for the purposes of the present invention, can be defined as the wavelength at which the spectrum power drops below a predetermined value (such as 20 dB, or 30 dB). The actual “edge” may be different for different applications. In frequency metrology applications, where the goal is to compare a stabilized continuum frequency comb to another light source that lies outside the spectral bandwidth of the comb, the current approach (harmonic generation) requires that a portion of the frequency comb be “frequency doubled” so that it overlaps with the wavelength to be measured. While this arrangement produces the desired result, it requires the use of additional nonlinear elements.
A method of extending the continuum—even to a narrow range of frequencies—without needing additional nonlinear elements would be considered a significant advance in the art. Inasmuch as the short wavelength edge of a continuum is usually limited by the large material dispersion of the waveguide medium itself, attempts to extend the continuum in this direction by merely increasing the pump power have been found futile. Further, in situations where the continuum includes harmonics far from the continuum, it would be beneficial to be able to “insert” one or more peaks along the spectrum between the harmonics and the continuum.
Thus, a need remains in the art for an arrangement capable of generating light pulses outside of the generated continuum (or in spectral regions where the continuum generation is very weak) without resorting to the inclusion of additional nonlinear components.